1. Field of the Invention
The present invention relates to a closure for containers and a sealing element for the closure.
The closure and sealing element in accordance with the present invention have suitable pliability derived from their molding materials; and furthermore, they have excellent thermal resistance, superior gas barrier properties and good oil resistance. Therefore, the closure and sealing element have such good mountability onto (or within) containers and such excellent sealing properties of the containers that the closure and sealing element procure long-term shelf stability.
2. Related Art of the Invention
As conventional molding materials for producing closures for containers (for example, caps, stoppers and the like) and sealing elements (packing elements) for the closures, generally, use has been made of resins with suitable pliability, such as vinyl chloride resins and low-density polyethylenes. As the molding materials, furthermore, use has been made of resin compositions with pliability provided by blending an elastomer or a softening agent to polyolefin resins such as high-density polyethylenes and polypropylene.
Alternatively, Japanese Patent Application Laid-open No. Hei 5-295053 describes that a composition for a sealing material, comprising 100 parts by weight of a block copolymer containing a polymer block of an aromatic vinyl compound and an isobutylenic polymer block and 0 to 80 parts by weight of other blending agents, has greater degrees of gas barrier properties and pliability derived from the block copolymer and that the composition is used for example as a lid member for sealing bottles.
Furthermore, Japanese Patent Application Laid-open No. Hei 5-212104 proposes to produce sealing articles for medicinal and medical products using a block copolymer containing a polymer block of an aromatic vinyl compound and an isobutylenic polymer block. These sealing materials have been produced from rubber materials, conventionally, which are illustrated for example as rubber stoppers for pharmaceutical agents, rubber stoppers for blood sampling tubes, and syringe caps working also as containers for filling pharmaceutical solutions. The Japanese Patent Application Laid-open No. Hei 5-212104 describes that the medicinal and medical sealing articles produced by using the block copolymer are excellent in terms of pliability, gas barrier properties, non-solubilizability and the like and that a thermoplastic polymer such as a block copolymer of ethylene and propylene may possibly be mixed with the block copolymer.
However, the closures and sealing elements comprising vinyl chloride resins may potentially contaminate the contents due to the bleed out of the remaining monomers and plasticizers under some conditions of the contents and the environment for use. Because the closures and the like comprising low-density polyethylenes have poor thermal resistance and gas barrier properties, they may possibly be deteriorated through the thermal treatment after filling and sealing. Additionally, they have poor long-term shelf stability. The closures and the like comprising a resin composition of a polyolefin resin blended with a softening agent may potentially contaminate the contents due to the bleed out of the softening agent under some conditions of the contents, the environment for use and the like. Still furthermore, the gas barrier properties of the closures and the like comprising a resin composition of the polyolefin resin blended with an elastomer are in general distinctively decreased due to the blending of the elastomer. Therefore, such closures and the like do not have good long-term shelf stability. Additionally, the blending of an elastomer alone cannot often provide satisfactory pliability. In such case, a softening agent should be used in combination, involving the contamination problem described above.
Alternatively, a sealing material comprising as a principal component a block copolymer containing a polymer block of an aromatic vinyl compound and an isobutylenic polymer block, as is described in Japanese Patent Application Laid-open Nos. Hei 5-295053 and Hei 5-212104, has higher pliability together with excellent gas barrier properties. The present inventors have made attempts to mold a closure and a sealing element for the closure by using the sealing material to seal a container by using them. However, it has been found that when mounting a threaded cap integrally molded from the sealing material onto a container, the slide of the cap in contact with the container is so poor because of too high pliability of the cap that excess strength is needed for the mounting thereof. Furthermore, it has been found that when a threaded cap mounted with a packing element molded from the sealing material is to be mounted onto a container, the slide of the cap on contact to the container is so poor because of too high pliability of the packing element that the back motion readily occurs, whereby complete sealing is not readily achieved. Furthermore, the oil resistance of the sealing material is not so high. Based on these findings, the inventors have found that the closure and sealing element comprising a material principally comprising the block copolymer are applicable in a narrow range of fields in a practical sense and that the closure and sealing element are only used for specific applications.